Process for dephosphorizing and desulphurizing steel



Patented F b. 14, 193a UNITED STATES 2,141,; PRDCESS FOBDEPHOSPHORIZING AND DESULPHURIZING STEEL PaulLemoinc, Paris, France, assignor to Socit dElectrochimi, dElectromtallnrg'ie et des Aciries Electrlqnes. d'Uginc, France, a corporation of France Pal-in No Drawing. ApplicationApril 2, ms, sdu

5 Claims.

The present invent-ion has for subject a process for the far reaching dephosphorization of steel, accompanied by desulphurization 01 this metal, with the aid of one or more alkaline elements and of one or more oxidizing elements, making use for dephosphorization of the known afllni'ty of alkaline bases for phosphoric anhydride and ior desulphurization, of the known reaction of formation of alkaline sulphides.

\ -In the case of steel, treatment by alkaline bases or salts which theoretically should cause the removal of sulphur and phosphorus, has never led in practice to this latter result; at the high temperature of the liquid steel it is impeded by the immediate volatilization of the alkaline elements before these have been able to exercise a practicalaction upon the steel. It is thus that the known desulphurizing action of sodium carbonate upon cast iron has never been successfully extended to steel.

Now according to the present invention it has been discovered that there can be obtained a strong dephosphorization of steel accompanied by desulphurization by putting into contact, so

25 that they act at least partly below the surface the liquid metal, a substance or a mixture of substances in the state of powder or fine grains, agglomerated or not, responding to the following conditions:

1. Of evolving in contact with the steel gases or vapours producing aviolent agitation and a seething of the molten bath formed by the metal and the added substance or substances. v

2. Of containing an alkaline base or bases such 35 atsa soda, potash, lithla, in the free or combined s te.

3.Of containing beside the carbonic acid of the alkaline carbonates-if such alkaline compounds are employed as are to be recommended-- 40 a relatively considerable proportion of an element which is oxidizing with respect to the phosphorous and the more oxidizing the less dissolved oxygen the metal contains.

Due to the employment of this oxidizing ele- 45 ment it is observed not only that there is not complete volatilization of the alkaline element added as was produced in experiments made heretofore, but that there is formed at the surface a slag containing alkaline bases and phosphoric 0 acid, and that the eiiect of dephosphorizatlon is obtained at the same time as a desulphurl'zation. This combination of means allows thus the theoretical action of the alkaline bases to be practically ensured which is normally prevented by 55 the volatility thereof. i

' It is possible to flnd combined in single substances the qualities required for the conditions enumerated above, for, example in sodium chlorate, but such a substance would evolve disagreeable vapours oi chlorine. Also it is economical In Italy April 11, 1935 and preferable to utilize mixtures of substances. It is particularly simple and economical to apply mixtures with a basis of sodium carbonate. Sodium carbonate alone does not give any practical result,'but very remarkable results are obtained bythe concomitant action of an oxidizing element. It has been observed that when the steel to be dephosphorized contains relatively little dissolved oxygen it is useful to employ energetic oxidizing agents-as for example barium dioxide or potassium permanganate or manganese dioxide. *By way of example in all practical cases excellent results are obtained with a mixture of 10 parts by weight of sodium carbonate and 3 parts of barium dioxide.

When the metal to be dephosphorized contains much dissolved oxygen, as for example overblown Thomas steel, which it is desired to cause to undergo a suplementary dephosphorization, it is observed that a smaller quantity of strongly oxidizing elements may be used, or even less energetic oxidizing may be used, like metallic oxides reducible by phosphorus and in particular iron oxide. The steel may contain in this case without disadvantage for dephosphorization manganese even in quite' considerable quantities.

It is obvious that the feature of adding bodies other than the essential alkaline or oxidizing substances, for example magnesia, 'fluorspar, lime, etc., does not lie outside the scope oi the invention.

- Likewise the feature of using alkaline and oxidizing substances otherwise than in admixture does not fall outside the scope of the invention, although it is preferable to make as intimate a mixture of them as possible. Finally any known mixture does not fall outside the scope of the invention which, brought to the high temperature of the steel, would give by chemical reaction at the same time an alkaline base or bases and an oxidizing element.

The bringing into contact of the metal and the reacting substance or mixture takes place under the general condition stated above, for example by. gradually introducing this body or mixture in powder into the jet of liquid steel while causing it to run into a casting ladle or any receptacle, for example into a furnace. The sub? stance or mixture may likewise be introduced into the ladle while the jet of metal is pouring therein, or even, especially in the case oi small quantitles, the substance or mixture may be placed at the bottom of the ladle before pouring themetal, but in this latter case the pouring will be performed with care so as to avoid any excessive seething. One may also make use of briquettes containing the reactants well mixed. The three above means lead to a very rapid reaction since the dephosphorization and possibly the desulphurization terminate as soon as the seething has finished, that is to say a little after the pouring of the metal into the ladle. The three procedures set forth above are intended to be covered within the scope of the term introducing below the surface of the metal, used in the appended claims.

The application of the process has for result not onlya very far reaching dephosphorization and desulphurization and in certain cases of a very great rapidity, but also a very great regularity in the degree of dephosphorization anddesulphurization from one operation to another, all things being equal, and this with .a remarkable automaticity without employment of a control during the operation.

The choice of the various ingredients and their proportions vary according to the type of steel, the permissible cooling, and the quantities of phosphorus and sulphur to be eliminated.

Thus the greater the content of phosphorus the greater will have to be the quantity of the mixture employed or the more reactive the mixture will have to be.

In the same way the mixture may be caused to vary according to the quantity of oxygen dissolved in the metal by causing to act for example as has been indicated above, an oxidizing element the more active or in larger quantity the lower is the quantity of oxygen dissolved .in the metal.

Finally account must be taken of the temperature of the metal. If for exampleone is concerned with a steel containing much dissolved oxygen, not very far from the solidification point the cooling of which should be avoided, there will be employed in the mixture an energetic oxidizing element ,instead of employing iron oxide, which would be sufilcient however to obtain the dephosphorization if the metal was maintained by any means in the molten state. i If it is desired to avoid an excessive cooling of the steel, during the operation of dephosphorization and desulphurization, an evolution of -heat may be produced in the receptacle where the steel is poured, by the application of mixtures evolving heat by their combination, for example alumino-thermic or silico-thermic mixtures.

A simple method consists in mixing for this purpose a reducing agent like silicon for example and sodium carbonate. The silicon may be added to the dephosphorizing mixture itself, for example in the proportion of one part by weight of Si for ten parts of NazCO3 and three parts of BaOz.

It is very remarkable that in this case, in spite of the employment of Si, there is still a very considerable dephosphorization. However, if the quantity of Si is increased, dephosphorization becomes less complete while on the other hand desulphurization increases. One may even with a sufii'cient content of silicon have practically nothing more than desulphurization.

'The presence of manganese does not hinder the dephosphorization nor the desulphurization.

The following are examples of the carrying out of the process forming the subject of the invention.

Example 1 Dephosphorization of 0.060 to 0.025% Desulphurization of 0.055 to 0.030% with, per

metric ton of steel:

5.000 gm. of NazCOa 1.650 gm. of 38.02

Example 2 In the two examples above the mixture, prepared in advance, of these two bodies was projected into the jet of metal during and in proportion to its pouring.

What I claim is: v

1. A process for the simultaneous dephosphorization and desulphurization of steel consisting in pouring the metal in the liquid state into a ladle and introducing below the surface of the metal and avoiding alkali volatilization a mixture of a substance consisting of a basis of an alkali metal carbonate in major proportion and an energetic oxidizing agent of the group consisting of barium dioxide, potassium permanganate and manganese dioxide said mixture having the property of melting and reacting so quickly without substantially-increasing the temperature above the dephosphorization point so as to ensure at the end of the pouring a substantial dephosphorization and desulphurization of the steel, said oxidizing agent being present in sufiicient amount to produce material oxidation of phosphorus.

2. A process for the simultaneous dephosphorization and desulphurization of steel containing substantially dissolved oxygen, consisting in pouring the metal in the liquid state into a ladle and introducing below the surface of the metal and avoiding alkali volatilization a mixture of substances consisting of a base of an alkali metal carbonate in major proportion and a metallic oxide reducible by :phosphorus, said mixture having the property of melting and reacting so quickly without substantially increasing the temperature above the dephosphorization point so as to ensure at the end of the pouring a substantial dephosphorization and desulphurization of the steel, said metallic oxide being present in sufiicient amount to produce material oxidation of phosphorus.

3.-Process as claimed in claim 2 in which the metallic oxide is iron oxide.

4. A process for the simultaneous dephosphorization and desulphurization of steel consisting in pouring the metal in the liquid state into a ladle and introducing below the surface of the metal and avoiding alkali volatilization a mixture of a substance consisting of a base of an alkali metal carbonate in major proportion an energetic oxidizing agent of the group consisting of barium dioxide, potassium, permanganate and manganese dioxide, and a metallic oxide reducible by phosphorus, said mixture having the property of melting and reacting so quickly without substantially increasing the temperature above the dephosphorization point so as to ensure at the end of the pouring a substantial dephosphorization and desulphurization of the steel, said oxidizing compounds being present in sufficient amount to produce material oxidation of phosphorus.

5.- Process as claimed in claim 1, in which a metallo-thermic reaction evolving heat is performed.

ROBERT PAUL LEMOINE. 

